1. Field of the Invention
The present invention relates to multiplexers and demultiplexers for fiber optic networks. More particularly, the invention relates to a narrowband optical fiber wavelength division multiplexer/demultiplexer using twisted and fused optical fibers for enhancing fiber optic system design and flexibility.
2. Description of Related Art
Fiber optic wavelength-division multiplexers ("WDMs") have become one of the key components in all optical fiber systems. The reason for this is that WDMs enhance system design and flexibility, as they permit more signals to be transmitted and received over a single optical fiber. Some of the major advantages of WDM fiber optic systems are: (1) increasing transmission capacity per fiber, (2) reducing cost of the system, and (3) expanding channel services after installation of the optical fibers. As a result, WDMs are widely applied in various fields of optical fiber networks and systems.
WDMs function to multiplex ("MUX") and demultiplex ("DEMUX") the optical signals. There are two basic WDM systems in the field: uni-directional and bi-directional. In the uni-directional system, WDMs are used to MUX the signals at the transmission end, while DEMUXing the signals at the receiving end. In bi-directional systems, WDMs are used to MUX and DEMUX the signals at both the transmitting and receiving ends.
There presently exist fiber optic WDMs that use fused pairs of optical fibers aligned in parallel with one another. Such WDMs are capable of MUXing and DEMUXing two preselected wavelength lights, operating at wavelengths of 1310 nanometers ("nm") and 1550 nm. In these WDMs, two optical fibers are aligned in parallel and simultaneously fused together while being heated. The resultant fused optical fibers form a fiber optic coupler.
In parallel fiber fused WDMs, however, the optical fibers are not twisted. Absent twisting, when the parallel optical fibers are pulled apart while being heated in order to fuse them, a force must be applied to urge the fibers toward one another. Such WDMs thus require a machine or device to force the two fibers together during the fusing process, adding expense to the fabrication process. Furthermore, previous WDMs, such as parallel fused optical fiber couplers, have failed to accomplish MUXing/DEMUXing of lights having closely spaced wavelengths (i.e., narrowband MUXing/DEMUXing) with adequate isolation between the wavelengths in the 1500 to 1600 nm range. Erbium-doped amplifiers operate in this range. As a consequence, previous fused WDMs have not been suitable for use in a narrowband optical fiber network employing Erbium-doped amplifiers.
Other existing WDMs include grating or multilayer dielectric thin film filters. Such WDMs use lenses and filters to attain the MUX and DEMUX functions. These WDMs, however, are expensive because they incorporate an assembly of separately fabricated parts, and they are not as flexible nor as small as fused optical fiber WDMs. Moreover, they are not as reliable as fused WDMs. A prism can also be used for WDM, but practical realization is hampered by low dispersion.
Therefore, a need exists for a twisted optical fiber narrowband WDM that is inexpensive and reliable and can operate to MUX and DEMUX optical signals at narrow bandwidths.